The invention relates to an improved method for the production of methanol, olefin, and hydrocarbons from a methane-containing gas, such as natural gas. In particular, the invention integrates a hydrocarbon synthesis unit with a methanol synthesis unit.
Methanol is a major chemical raw material. Present global consumption is about 27 million tons per year. Major uses of methanol include the production of acetic acid, formaldehyde, and methy-t-butylether. The latter, an oxygenate additive to gasoline, accounts for about a third of all use.
Worldwide demand for methanol is expected to increase as much as five fold over the next decade as potential new applications become commercialized. Such applications include the conversion of methanol to gas, such as the Mobil MTG Process, the conversion of methanol to light olefins, the use of methanol for power generation, and the use of methanol for fuel-cell powered automobiles.
Methanol synthesis is based on the equilibrium reactions of syngas, reactions (1) and (2).
CO+2H2⇄CH3OHxe2x80x83xe2x80x83(1)
CO2+3H2⇄CH3OH+H2Oxe2x80x83xe2x80x83(2)
Syngas is defined as a gas comprising primarily carbon monoxide (CO), carbon dioxide (CO2) and hydrogen (H2). Other gases present in syngas include methane (CH4), and small amounts of light paraffins, such as ethane and propane. One way of characterizing the composition of a syngas stream for methanol synthesis is to account for the CO2 present in the syngas stream. The syngas number (SN) is defined as follows:
SN=(H2xe2x88x92CO2)/(CO+CO2)
The forward reactions (1) and (2) are exothermic, that is, they result in the formation of net heat. Also, the forward reactions (1) and (2) generate less volumes of MeOH (gas) than the volumes of feed (gas) used to form the methanol. Therefore, to maximize methanol yields, i.e., force reactions (1) and (2) to the right, the process requires low temperatures and high pressures for high conversion. Still, a typical methanol reactor will convert only about 20% to 60% of the syngas fed to the reactor in a single pass through. To obtain higher conversions the unreacted syngas is separated from the product methanol and recycled back to the reactor or directed to a second reactor to produce additional methanol.
The initial step in the production of methanol is to produce syngas from a methane-containing gas, such as natural gas or refinery off-gas. The associated costs of producing the syngas accounts for over half of the capital investment in the methanol plant. The syngas can be generated using steam methane reforming or partial oxidation reforming, which includes combined reforming or autothermal reforming process.
UK Patent Application GB 2092172A recognizes that partial oxidation reformers used in the production of syngas for the production of synthetic hydrocarbons, that is, Fischer-Tropsch type conversion, often produces an excess quantity of CO2 that eventually must be removed from the process stream. Consequently, associated costs in producing and removing the CO2 exist. The UK Patent Application teaches that the excess CO2 produced by the partial oxidation reformer can be utilized in part by first passing the syngas to a methanol synthesis reactor prior to the hydrocarbon synthesis reactor. The methanol synthesis utilizes the CO2 as a carbon source to produce methanol according to reaction (2). Alternatively, the CO2 can be mixed with hydrogen, produced from an external source, to convert the CO2 to more CO according to the water-gas shift reaction. The additional CO is then used to produce more synthetic hydrocarbon.
U.S. Pat. No. 5,177,114 to Van Dijk et al. teaches the conversion of natural gas to methanol or methanol and synthetic hydrocarbons using a relatively low-cost, self-sufficient process. The natural gas is mixed with a 1:1 O2/N2 stream at elevated temperatures and pressures to produce a reform gas, which is then used to produce methanol and/or synthetic hydrocarbons. The natural gas is converted without the need for a costly steam reformer or a partial oxidation reformer. Also, the process is directed to low carbon conversions, e.g., about 50 to 65%, so that the tail gas from the process can be used to drive the compressors and other energy intensive units in the process.
It is very likely that the world demand for methanol will increase five-fold over the next decade. Methanol will be used as a chemical feedstock and as a competing fuel for transportation and power generation. As a result, processes designed to produce methanol in an economically efficient manner are highly desirable.
The invention integrates a methanol synthesis process with a hydrocarbon synthesis process. Particularly, the invention combines a syngas stream from a steam reformer with a syngas stream from a partial oxidation reformer to take advantage of their respective product syngas compositions. The invention utilizes most of the CO2 and H2 produced by the reformers. The hydrogen is used to convert the excess CO2 to methanol and/or as an internal hydrogen source to further refine synthetic hydrocarbon. In the latter a portion of the hydrogen is separated from the first syngas stream or the recovered unreacted syngas stream, i.e., the recycle loop, and directed to a hydrocarbon synthesis refining unit. The combined syngas stream to the methanol reactor should have a SN of from 1.4 to 2.4, preferably of from 1.8 to 2.2.
In one embodiment, the invention further comprises separating a portion of CO2 from a product gas from the hydrocarbon synthesis reactor to form a CO2 containing gas and directing the CO2 gas to a unit selected from the steam reformer, the methanol synthesis reactor, the partial oxidation reformer, or any combination thereof Preferably, the separated CO2 is directed to the methanol synthesis reactor. In another embodiment, the invention further comprises directing a portion of the product gas from the hydrocarbon synthesis reactor without CO2 separation to a unit selected from the steam reformer, the methanol synthesis reactor, the partial oxidation reformer, or any combination thereof
The methanol synthesis reactor can be a low pressure reactor with an operating pressure from 200 psi to 700 psi or a conventional high pressure methanol synthesis reactor with an operating pressure from 500 psi to 2000 psi. The unreacted syngas from the methanol synthesis reactor can be separated from the methanol product and directed back to the methanol synthesis reactor, i.e., recycled, such that additional methanol can be produced. Alternatively, unreacted syngas can be directed to a secondary methanol synthesis reactor. In one embodiment, the methanol synthesis reactor will be a low pressure reactor, and the secondary methanol synthesis reactor a conventional high pressure reactor.
The produced methanol from the invention can be used to make olefins. In one embodiment, the produced methanol is first directed to a methanol refining unit where a portion of the water and other oxygenates are removed. The refined methanol is then used to make olefins, particularly ethylene and propylene. Preferably, a molecular sieve catalyst, more preferably a silicoaluminophosphate catalyst selected from SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, the metal containing forms thereof, and mixtures thereof, is used to convert the methanol to olefins.